# Running hot & Over heating @ interstate speeds

## Recommended Posts

Friday I leave SC on a 4hr trip to North GA to kayak & fish. About 2hr interstate, and 2hr secondary highways. 1hr from home I notice the Comanche is sluggish. I pull off at a rest stop, and instantly see steam, and hear the hiss. No light on the dash!

Thankfully it's only a short hose between the heater bypass valve, and the heater core. (The valve is apparently not closing completely ) I just looped the heater hose around and bypassed all of it. Top her off with water, and down the road we go. 45min later as we exit the interstate I see steam again! 🤬 This time it isn't just hot, it's over heating. Let it cool, fill it up, check for leaks, let it run, check for leaks. Nada! So I think I might have had an air pocket.

We continue on for 1hr on secondary roads, and highways without incident. Back on the interstate again, and I pull over after 30 min to check it. Yep it's hot again. I base this on 25yrs as an auto tech, and literally being able to feel the excessive heat coming off the motor. I let it cool, and get underway with a new route avoiding interstates. No more issues. So my 4hr trip took 7.5hr.

No issues Saturday while roaming the mountains of GA kayaking & fishing.

1hr down the interstate today, and it's over heating again! 🤬🤬🤬🤬

New route home, and 6hr later were here.

Now I've not checked the system with a pressure tester, or chem tested for a head gasket. I do know the water pump is pumping, radiator & cap is new, E cooling fan is working, thermostat is opening\closing, and system has no air in it.

I was towing a 500# trailer with 2 kayaks at around 50# & 75#, with around 200# of cargo in the bed.

It seemed like every time we would start running up long grades on the interstate it would get hot. It was fine on the long grades, and winding up mountains though. Only difference I see is the speeds were above 65mph on the interstate, and below 60mph on the highways.

I'll be doing a full diagnosis tomorrow, including why my temp light works with the ignition start, but never came on when it got hot, or over heated!

##### Share on other sites

Sounds similar to mine. New coolant, burped a few times, new cap, thermostat, some snake oil to help it cool more effiecently. Mine gets a bit over 210 on the dash and a temp gun to the head says that's about 220 going up grades and more recently when I was in Idaho in 80* weather on the fwy put an end to my moab trip since I couldn't even crawl there.

I figure part if my issue is the 3 electric fans, so I'm in the process of going to mechanical yet I have a 99 wj motor that has a different fan clutch right of the crank, and I have to male a new shroud. Hopefully you grt it figured out and I'll be watching this for reference.

Sent from my SM-G900V using Tapatalk

##### Share on other sites

Overheating at higher road speeds is almost always a sign of one of two things:

1.  poor water circulation due to a worn water pump

##### Share on other sites

25 minutes ago, AZJeff said:

Overheating at higher road speeds is almost always a sign of one of two things:

1.  poor water circulation due to a worn water pump

Or

3. a lower (suction) radiator hose that collapses because it doesn't have the internal spring.

##### Share on other sites

First thing you should do is junk the idiot light gauge cluster.

##### Share on other sites

The houses are new, water pump is new ish, and I'm going to do a flush first thing.

I have several kitchen oven thermometers that I use to find radiators with poor flow.
Carefully stuck them through the radiator in a zig-zag and let the engine reach NOT. Check your temps, and it will be obvious the one that's usually cooler by 30deg or more.

I actually had just ordered a gauge cluster, and sensors to work the gauges. Should all be here today!

##### Share on other sites

You say newish water pump, but don't say anything about the thermostat?

I had a few over heating issues a couple years back and the thermostat was part of the problem - but not a.ll the problem.  I did not burp the heater core, so had air in the system for a long time.  Also wound up replacing the radiator as part of my solution along with new hoses and a radiator cap.  After that was done, the truck was reasonably cool.  Only time the temp climbed was a very long grade (like sea level to 4100 feet) or with a 3000 lbs load in stop and go freeway traffic.

##### Share on other sites

You say newish water pump, but don't say anything about the thermostat?

I had a few over heating issues a couple years back and the thermostat was part of the problem - but not a.ll the problem.  I did not burp the heater core, so had air in the system.

First post: I covered the thermostat.

Heater core is completely bypassed, and system was properly filled.

I'll know later. I'm going to finally get it in my shop to check it.
##### Share on other sites

Since I don't know, is it opened or closed cooling system?

##### Share on other sites

One of my jeeps in the shop was having same issue. So we replaced the 31 year old radiator with a all metal 3 row core, put a new water pump, all new hoses, new thermostat, and went from mechanical fan and 1 electric fan to a 3 fan setup. Now the thing runs nice, about 195 on the highway at 75 plus mph and under 40 it runs right at the 203 degree mark.

Since we work on vehicles here labor is my time and the cost of all parts was 175 dollars (ebay metal 3 row core with 3 fans was 115). We use the 30 coolant 70 water mix with 2 bottles of water wetter.

As everyone said, check flow on radiator including thermostat for flow, then also make sure hose collapse is not a issue.

##### Share on other sites

I went through that problem with overheating only at speed and/or going up long grades. New water pump, radiator, hoses, stat. etc. etc. Turned out the cat was clogged nearly shut, discovered it when we were at a friends shop running a new exhaust system. Replaced it with one of the below - no problems since.   ROLL TIDE

##### Share on other sites

steam? hissing? from where? is that an open or closed system?

##### Share on other sites

Since I don't know, is it opened or closed cooling system?
Closed
##### Share on other sites

steam? hissing? from where? is that an open or closed system?
First time was the pin hole blow out in the heater hose. Second time was the cap.
##### Share on other sites

I went through that problem with overheating only at speed and/or going up long grades. New water pump, radiator, hoses, stat. etc. etc. Turned out the cat was clogged nearly shut, discovered it when we were at a friends shop running a new exhaust system. Replaced it with one of the below - no problems since.   ROLL TIDE

PO head already done a hack job on the exhaust. Cut 4in behind the coupler flange, 4 flared adapters, a 24in pipe, 2 flare adapters, a muffler from something huge (3.25") , and a 45deg turn down right in front of the axle. It was held together by about 8 u-bolt clamps, and hanging from 2 clothes hangers.

I drove it 3hr home from purchasing it, and the exhaust literally fell off along the way!

One of my first repairs was an OE replacement cat back exhaust. I reused the 24in pipe, and welded everything together, hung on new rubber.
##### Share on other sites

I filled & pressure tested the cap & system. The water pump had a very small leak on the gasket. I chem checked the system for a head gasket leak, and got negative results thankfully. Gave it a quick flush, and pressure tested again to verify.
I'm not gonna replace the gasket when a new pump is only $35, so I put in a new pump. I'll hang on to the old one as a spare in case I get in a jam, or someone needs one to get out of a jam. I put a new thermostat in, replaced all the heater hose, checked the radiator hoses. They do seem to be very new, but the lower hose doesn't have a retention spring to stop it from collapsing. I called NAPA, Oreilly, and AutoZone and none of their replacement hoses have the retention spring. Not sure about what to do there...... New temp sender for the gauge cluster. I figured out that the PO had never put the correct sender in when he put the idiot light cluster in! Guess that explains why I never got an idiot light when it ran hot\over heated. I would imagine that means I wasted money on a new oil pressure sender for a gauge. I'll finish tomorrow morning when it's cooler. I still need to check the radiator for flow, and check the E-fan to be sure it hasn't crapped out on us. ##### Link to comment ##### Share on other sites Most modern hoses don't have a spring. It's not how they are made anymore, the rubber is thick enough and has a strong enough braid in it to not need a spring. No issues with my springless hoses. ##### Link to comment ##### Share on other sites Most modern hoses don't have a spring. It's not how they are made anymore, the rubber is thick enough and has a strong enough braid in it to not need a spring. No issues with my springless hoses. I wasn't sure. I still see allot of heavy truck, and commercial engines that have retention springs, so I didn't want to assume that was the case. ##### Link to comment ##### Share on other sites Are you absolutely sure it is actually overheating? One thing to check before spending any more money: check to see if your coolant temp sender (the one that runs the gauge which is located at the back of the drivers side of the head) is original or if it looks like it's been replaced. In my experience, NONE of the aftermarket sensors read correctly; they all read high. Next time it starts rising, pull over and check the engine in multiple spots (back of head where sensor is, middle, by water pump, by thermostat, etc.) with a infrared thermometer. I also recommend an inline hose temp sensor and gauge which will give you a more accurate idea of what's going on. Get one of these - they are cheap like 10 bucks or so: ##### Link to comment ##### Share on other sites Are you absolutely sure it is actually overheating? One thing to check before spending any more money: check to see if your coolant temp sender (the one that runs the gauge which is located at the back of the drivers side of the head) is original or if it looks like it's been replaced. In my experience, NONE of the aftermarket sensors read correctly; they all read high. Next time it starts rising, pull over and check the engine in multiple spots (back of head where sensor is, middle, by water pump, by thermostat, etc.) with a infrared thermometer. I also recommend an inline hose temp sensor and gauge which will give you a more accurate idea of what's going on. Get one of these - they are cheap like 10 bucks or so: I'm absolutely sure it's running hot. That's any condition causing the coolant to heat up above the NOT. It only over heated once. That's the point at which the coolant boils, and exceeds the limits of the radiator cap. The cap releases the excess pressure, and allows the coolant & steam to escape. I'm not being a smart @$\$, but those are two separate conditions.

I use my multi meter temp probe, or my temp gun to watch the temp while I'm testing.
I found my last 2 problems today.

I just swapped out my idiot cluster to a gauge cluster today. I also found the gauge sensor with the plug broken. Gotta get one in the AM.
I noticed that the electric fan only ran a short time. So I find the fan would run for a minute, then abruptly stop. It would also spin a half turn then stop. It ran a full cycle once.
Crazy, but the first time I checked it, the fan ran it's cycle, so I assumed it was working properly.
##### Share on other sites

Replaced the coolant temp sender for the gauge. It now reads properly.

Replace the fan relay, and checked the fan operation again. Then replaced the electric fan. The fan still stalls, and randomly will run a cycle. I replaced the coolant temp sensor with a spare to rule it out. Same issue with the fan. I've also checked the plug terminals.

I know this truck was wrecked in the front, so I imagine the problem is going to be some crazy issue from a crappy repair by the PO.

Any ideas?

##### Share on other sites

Any idea how clean the block and radiator are? I picked up some thermocure and just flushed mine. 1) I wish my license wasn't suspend so I could more than let it idle and risk a few laps around the block 2) I should have left it in a few days but I leave for a job outta town tomorrow and was anxious. Anyways, this is what my first flush looked like. Supposedly it dissolves rust and corrosion and the byproduct is a block soot. I need to seal up my homemade shroud but so far my temps have dropped, other than idling for about an hour, then it wants to creep past 210.

If you do try this stuff, when you do the 2 flushes before filling with coolant drive ot, don't be like me and mostly idle. The longer you leave it in the better it works from what I gather. I forgot the link but some classic car restorer has his customers drive up to 6 weeks with this in the system for really neglected rigs.

Sent from my SM-G900V using Tapatalk

##### Share on other sites

I usually refer to such products as "Snake Oil". I've not heard of it before so I'll give due diligence and research.

I generally do not use chemical flush additives. I have made exceptions....

I know I'm old school, but I was taught that if it's abrasive or corrosive enough to clear away rust & corrosion, it's not likely doing anything good for your gaskets & seals!

##### Share on other sites

I know this truck was wrecked in the front, so I imagine the problem is going to be some crazy issue from a crappy repair by the PO.
Any ideas?

So I'm likely to run a new power\ground from the fan relay to the fan. I'm not spending the time & effort to locate the problem I know exist!
When tested, the fan is only seeing 8-9vdc, and the resistance in the wire shoots up.
##### Share on other sites

44 minutes ago, GreasemonkeySC said:

So I'm likely to run a new power\ground from the fan relay to the fan. I'm not spending the time & effort to locate the problem I know exist!

It's your truck - go for it. Then the next guy who gets your truck will be cursing the PO's workarounds. Not trying to be as azz, but I'd take the time to find the cause of the fan voltage drop.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.